1. Field of the Invention
The invention relates to a process for the production of a composite article of at least two component pieces, bonded firmly to one another, of a vulcanized rubber compound on the one hand and a polyamide composition selected from the group consisting of a polyamide, a polyamide molding composition, a polyamide blend or a fiber reinforced composite material with a polyamide matrix on the other hand. The invention also relates to the composite article produced by this process.
2. Discussion of the Background
The technical object of firmly bonding rubbers and thermoplastics to one another is old and to date has been achieved in various ways, which overall, are unsatisfactory.
Composite materials of thermoplastically rigid and elastomeric molding compositions are usually joined together by gluing, screwing, riveting or mechanical clawing, or using an adhesion promoter. Interesting processes have recently been developed for production of a bond between molding compositions based on polyphenylene ethers (PPE) and certain rubbers which can be vulcanized with sulphur or peroxide (EP-A-0 196 407 and EP-A-0 315 749).
The adhesive strength values achieved by such methods are considerable; nevertheless, the resistance of the PPE molding compositions to solvents is still just as unsatisfactory, as is their stability to weathering. Moreover, such molding compositions can be processed only with difficulty because of their high melt viscosity.
In theory other thermoplastic molding compositions, with improved physical properties, could be employed in this process. However, it has been found that, for example, with polyamides (PA), which are known to be resistant to solvents, adequate adhesion values cannot be achieved under the process conditions recognized as essential. It therefore did not seem possible to produce bonds between polyamides and rubbers which have an adequate adhesive strength and a good resistance to solvents and weathering.
A solution to this problem has been proposed in EP-A-0 344 427. A thermoplastic which comprises at least 30 wt % of aliphatic polyamides are employed in conjunction with a rubber composition based on a rubber which contains carboxyl groups, as the essential feature.
On the basis of this, components of polyamide molding compositions reinforced with continuous fibers and of rubber are described in EP-A-0 422 358. Here too, the essential feature is that the rubber used contains carboxyl or anhydride functional groups.
According to the prior art, composites are thus only obtained, without using adhesion promoters, if the rubber contains carboxyl or anhydride groups. Furthermore, the polyamide should contain more amino than carboxyl groups.
According to the present state of knowledge, the manufacturer of rubber/polyamide composites faces the following dilemma:
Either to use rubber compounds based on resins produced by the RFS process (RFS=resorcinol/formaldehyde/silicon dioxide), as described by W. Kleemann in "Mischungen fur die Elastverarbeitung (Mixtures for elastomer processing)" (VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig 1982). The flexability in choosing the physical properties of the elastomers is then limited, because flexible mixtures and mixtures with ethylene/propylene rubber adhere poorly. Other disadvantages of this method are described, for example, in Werner Hofmann, Rubber Technology Handbook 1989, Carl Hanser Verlag, chapter 4.7.4.
The use of external adhesion promoters by treatment of the surface of the plastic. This process is both cumbersome and environmentally polluting. This process is described, for example, in the journal "Kautschuk und Gummi-Kunststoffe", October 1991, pages 963 to 970.
The use of special rubbers with carboxyl or anhydride groups, as described in EP-A-0 344 427. The properties of these rubbers differ from those of standard rubbers. The special rubbers moreover are significantly more expensive than standard rubber. In addition, this method functions properly only if the polyamides used are regulated by diamines.
The use of peroxides and silanes in rubber compounds is known per se.
Peroxides are preferred vulcanization agents over sulphur or sulphur donors if a high resistance to heat is to be achieved in the vulcanized rubber compounds.
Silanes have been used in polymers such as rubbers, thermosets and thermoplastics if binding of inorganic fillers, for example talcum powder, quartz powder or glass fibers, to the organic polymer is to be effected. The manufacturers of fillers as a rule supply their products to the users ready-treated with silanes or other sizes. The users mix the fillers pretreated in this way, with the polymers so as to obtain reinforced molding compositions or reinforced compounds.
In the case of systems with a high content of mineral fillers, however, it may be advantageous for the user to employ non-sized fillers and to mix the silane or other sizes with the polymer before or together with the inorganic filler in the so-called "additive process". Such a procedure is often used for preparation of rubber compounds. These rubber compounds then contain in general about 0.2 to 2.5% by weight of silane, based on the filler, or up to about 1.5% by weight, based on the rubber compound (company publication "Dynasilan.RTM." from Huls AG, D-45764 Marl, Issue 7/92, page 26). A content of 2% by weight of silane in the rubber compound, based on the filler, is not usually exceeded.
Suggestions of the bonding mechanism between inorganic and organic components have been published in numerous publications. Reference may be made in particular in this context, to the company publication from Huls AG, D-45764 Marl, entitled "Anwendungen von organofunktionellen Silanen (Uses of organofunctional silanes)" (October 1989).
The object of the present invention was therefore to develop composite partners which allow the production of composite articles of polyamides and elastomers yet still allow for the formation of a firm bond, in which the following aspects should be met:
Polyamides, polyamide blends, polyamide molding compositions and fiber reinforced composite materials with a polyamide matrix should be functional as the polyamide component. A special treatment--for example replacement of carboxyl groups by amino groups--should not be necessary.
Adhesive resins which impair the properties of the vulcanized rubber compounds should not be used in the rubber compounds.
Adhesion promoters which have to be applied between the polyamide and rubber in a separate working step should likewise not be employed.
Commercially available standard rubbers should be able to be used.
Additives for the adhesive bond should not substantially impair the properties of the vulcanized rubber compounds.
It has now been found that the requirements mentioned above are met if the rubber compounds contain certain silanes and are vulcanized with peroxides.
In contrast to the prior art use of silanes, the silanes of the present invention are employed in a controlled manner in order to achieve firm bonding to the thermoplastic/vulcanized rubber compound interface. Larger quantities are required for this purpose than for mere binding of inorganic fillers and reinforcing substances. The fact that the addition of such silanes to achieve this object was not obvious can be seen merely from the fact that no satisfactory bond is achieved with other thermoplastics, such as, for example, aromatic polyesters, for example polybutylene terephthalate or polyethylene terephthalate, instead of polyamide.
There are still no confirmed findings on the bonding mechanism which is active here between the polyamide and vulcanized rubber compound.